The process is quite simple:
1) setup your subscription levels and enter your payment details (checks/money order/paypal).
2) make all your forums require a certain base access level (these will be for your paid subscribers)
3) make a new forum which guests and "non paid members" can browse.
4) put a sticky in that forum with the instructions and links to all the items you want those unpaid users to download/view.
register a new account, and test.
EDIT: I believe this setup is working quite well for these guys http://www.cessna-pilots.net/forums/
They've been around since at least 2006